Aqueous dispersion of intrinsically electroconductive polyalkoxythiophenes, a process for their preparation and their use

ABSTRACT

An aqueous dispersion is described which contains electroconductive, doped, polyalkoxythiophene which is soluble in aprotic organic solvents and is insoluble in water. The dispersion has a residual solvent content of less than or equal to 25% by weight, based on the total weight of the dispersion. A process for the preparation of this dispersion and the use of the dispersion for forming coatings on substrates are also described.

BACKGROUND OF THE INVENTION

There have been worldwide endeavors for some years to synthesizeso-called "intrinsically" electroconductive polymers. These are taken tomean polymer materials which have an inherent conductivity without theaddition of electroconductive substances such as metal powders orfibers, conductive black or the like. Examples of such polymers arepolyacetylene, polypyrrole, polythiophene, polyaniline,polyparaphenylene, polyphenylene sulfide, etc. However, polyconjugatedbonding systems of this type are only electroconductive in the dopedstate, i.e. they must be converted into a conductive state by means ofan oxidant or reducing agent in an electrochemical or chemical reaction.However, all of the materials listed above are insoluble and infusiblein the doped state i.e. are unsuitable for further processing.

Therefore, up to a few years ago, there were few concrete applicationsfor intrinsically electroconductive polymers. A further disadvantage wasthe low stability of the novel materials, in particular in moistatmospheres.

In order to obtain processable electroconductive polymers, soluble,intrinsically electroconductive polymers were developed (cf. R. L.Elsenbaumer, K. Y. Jen and R. Oboodi, Synth. Met. 15 (1986), 169). Inparticular, doped polyalkoxythiophenes synthesized by electrochemicalmethods are distinguished by high stability (M. Feldhues et al., Synth.Met. 28 (1989), C487). In doped form, these materials are soluble in lowpercentages in organic aprotic solvents, such as toluene, THF,acetonitrile, diethylformamide and N-methylpyrrolidone, and aretherefore suitable as base materials for the electroconductive and/orantistatic coating of substrates (EP-A-0 328 981, EP-A-0 257 573 andEP-A-0 328 982).

However, it has proven very disadvantageous in the production of suchcoatings on an industrial scale that organic solvents have to be used,since many organic solvents today require industrial post-combustion,which is very expensive and complex. Attempts are therefore increasinglybeing made, in particular for the continuous coating of flexiblesubstrates, such as, for example, films, to employ aqueous systems inwhich at least a large part of the organic solvent has been replaced bywater.

In principle, two methods are conceivable for obtaining aqueous coatingformulations based on intrinsically electroconductive polymers. Thefirst is the development of novel, water-soluble, conductive polymers(A. O. Patil et al., J. Am. Chem. Soc. 109 (1987), 1858; E. E. Havingaet al., Polymer Bulletin 18 (1987), 277; A. O. Patil et al., Synth. Met.20 (1987), 151). A second method is the preparation of aqueousdispersions. However, industrial processing of the materials, whichbecame known as a result of the new development of water-solublepolymers, has hitherto been unsuccessful due to only very low electricalconductivity values achieved of below 0.1 S/cm and due to poor stabilityin water.

S. P. Armes and M. Aldissi, Polymer 31 (1990), 569, describe thepreparation of an aqueous polypyrrole dispersion. In the processdescribed, a steric stabilizer (poly-2-vinylpyridylbutyl methacrylate)must be added in order to avoid particle agglomeration of thepolypyrrole, which can only be dispersed with difficulty. The specificconductivity of the dried powder is from 1 to 2 S/cm. No suitability ofthe material for coating purposes has been disclosed.

S. P. Armes et al., J. Coll. Interf. Sci. 118 (1987), 410, likewisedescribes an aqueous polypyrrole dispersion, in which the dispersionstabilizer employed is polyvinyl acetate. The individual conductiveparticles (not the liquid dispersion) exhibit a specific conductivity of5 S/cm at a particle diameter of from 100 to 150 nm. The properties ofthin layers made from these dispersions are not described.

DE-A-38 34 526 describes the coating of substrates with free-flowingdispersions based on polyaniline. It is also possible to use aqueousmedia here. It is disadvantageous that, after coating is complete andthe solvent has been removed, it is necessary to convert the layerchemically or electrochemically into the conductive form necessary forits later function.

In both cases, insoluble, intrinsically conductive polymers in the dopedform are used. Aqueous dispersions of conductive polymers which aresoluble in organic solvents have not been disclosed.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an aqueousdispersion of an intrinsically electroconductive polymer which requiresneither addition of stabilizers nor subsequent doping and which issuitable for the coating of substrates in order to achieve anelectroconductive and/or antistatic layer.

Another object of the present invention is to provide coated articleshaving an electroconductive and/or antistatic layer made by a processcomprising the steps of applying to a substrate an aqueous dispersion ofan intrinsically electroconductive polymer which requires neitheraddition of stabilizers nor subsequent doping.

It is also an object of the present invention to provide a bag withheat-sealed edges and, an antistatic layer made by a process comprisingthe steps of applying to one side of a film an aqueous dispersioncomprising electroconductive, doped, polyalkoxythiophene.

In accomplishing these objects, there has been provided in accordancewith one aspect of the present invention an aqueous dispersioncontaining, an electroconductive polyalkoxythiophene which is insolublein water, but soluble in dipolar aprotic organic solvents, the aqueousdispersion having a maximum residual solvent content of about 25%.

According to another aspect of the present invention, there is provided,a bag with heat-sealed edges, made by a process comprising the steps ofapplying to one side of a film comprising at least one functional layeron the second side, an aqueous dispersion comprising electroconductivedoped polyalkoxythiophene, to produce a coated film; laminating thesecond side of the coated film with a heat-sealable film to produce acoated laminated film; forming a bag by folding; and heat-sealing thebag with the coated side on the outside of the bag.

According to still another aspect of the present invention, there isprovided, a bag with heat-sealed edges, made by a process comprising thesteps of applying to one side of a film an aqueous dispersion comprisingelectroconductive, doped, polyalkoxythiophene, to produce a coated film;applying to one side of a heat-sealable film an aqueous dispersioncomprising electroconductive, doped, polyalkoxythiophene, and a suitablebinder, to produce a coated heat-sealable film; laminating the uncoatedside of said coated film with the uncoated side of the heat-sealablefilm to produce a coated laminated film; forming a bag by folding; andheat sealing the bag.

Further objects, features and advantages of the present invention willbecome apparent from the detailed description of preferred embodimentsthat follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preparation of suitable polyalkoxythiophenes is described in EP-A-0257 573. This application discloses intrinsically electroconductivepolymers, preferably oligomers, which are synthesized by electrochemicalmethods and, in the oxidized form, are fully soluble in dipolar aproticsolvents at room temperature and which comprise structural units of theformula: ##STR1## in which R₁ is a C₁ - to Cu₁₂ -, preferably C₁ - to C₄-alkoxy group. The monomer units are bonded to one another in the2-position and/or 5-position.

In the oxidized form, the electroconductive polymers contain thecorresponding number of anions, preferably anions of the conductive saltemployed in the electrolysis preparation process, to compensate for thepositive charges on the polymer chain. Examples are: BF₄ ⁻, PF₆ ⁻, SbF₆⁻, SbCl₆ ⁻, FeCl₄ ⁻ and [Fe(CN)₆ ]³⁻, etc.

In order to obtain the aqueous dispersions according to the invention,the conductive polymer is first dissolved in a dipolar aprotic solvent(for example N-methyl-pyrrolidone). In addition, one or morenonconductive binders are preferably added to this solution; thesebinders have the task of providing the later layer with mechanicalstability and better adhesion. These binders should likewise be solubleor dispersible in the same solvent and additionally in water. Thesolution is then added dropwise to water, the dispersion forming beingmixed permanently. If desired, additives (flow-control agents, wettingagents) can be added to the dispersion forming.

The dispersion prepared in this way has a solids content of from 2 to15% by weight, preferably of from 4 to 8% by weight, in each case basedon the total weight of the dispersion. From 5 to 100% by weight,preferably from 10 to 30% by weight, of the dispersion comprises theabove-described intrinsically conductive polyalkoxythiophenes, and from0 to 95% by weight, preferably from 70 to 90% by weight, of thedispersion comprises nonconductive binders and other additives. Thewater content is from 95 to 75% by weight, preferably from 80 to 90% byweight; the residual solvent content is between 5 and 25% by weight,preferably between 10 and 20% by weight.

The dispersion according to the invention is particularly suitable forcoating a very wide variety of substrates, preferably films made ofpolyester, polypropylene, polyvinyl chloride, polyacetate, polyethylene,polycarbonate and polyamide, by conventional methods of applying surfacecoatings, such as dipping, spraying and various printing processes, suchas, for example, application by a smooth roller, application by anengraved roller, intaglio printing and flexographic printing. Thedispersion according to the invention is particularly suitable for thecontinuous coating of film webs. In layers having thicknesses of lessthan 1 μm, preferably from 50 to 500 nm, antistatic, transparent layershaving surface resistances of between 10⁴ and 10⁹ ohms, preferablybetween 10⁶ and 10⁸ ohms (DIN 53482) and a transparency of between 50%and 98% (ASTM 1003-61) can be produced in this way.

Films provided in this way with an antistatic layer can advantageouslybe converted into composite films, preferably using a film made from aheat-sealable material, preferably polyethylene, and applied by adhesivelamination. A film composite of this type can comprise, for example, asubstrate film, for example a polyester film, which has been coated withthe aqueous dispersion according to the invention and has a thickness inthe range between 6 and 100 μm, preferably between 12 and 50 μm, and aheat-sealable film, for example a polyethylene film, having a thicknessof between 20 and 200 μm, preferably between 25 and 75 μm. Heat-sealedpackaging bags can then also be produced from a composite of this type,the polyester film coated with the dispersion according to the inventionbeing arranged on the outside of the packaging bag. For an applicationof this type, the heat-sealable film used can then also be polyethylenefilm provided with an antistatic finish by means of conventional knownagents.

The use of suitable heat-sealable binders also makes it possible for theheat-sealable film itself to be coated with the aqueous dispersionaccording to the invention and thus provided with an antistatic finish.In this way, heat-sealable composite films coated with an antistaticlayer on both sides are obtained; these can be converted, for example,into bags.

A further very advantageous application of the dispersion according tothe invention is the coating of metallized films. Commercially availablemetallized films, in particular metallized polyester or polypropylenefilms, are preferably produced by applying aluminum layers by vacuumvapor deposition in thicknesses in the range between 10 and 200 nm.Optically transparent metal layers having optical transparency valuesabove about 30% are achieved at layer thicknesses of less than 20 nm.The surface resistances of transparent layers of this type are betweenabout 30 and 200 ohms. Coating of a film of this type on the reverse,i.e. on the side not coated with the metallization, with the aqueousdispersion according to the invention and lamination of this film withthe metal side against a heat-sealable film, for example made frompolyethylene, gives a laminate which can be converted as described aboveto give heat-sealed packaging bags. The metal layer on the insideadditionally provides a screening effect against high-frequencyalternating fields.

The advantages of the antistatic layers applied with the aid of theaqueous dispersions described above compared with conventional,so-called chemical antistatic agents (for example fatty acid esters,quaternary ammonium salts, amines, etc.) are principally thesignificantly lower surface resistances (below 10⁹ ohms) and theindependence of the antistatic action of the ambient atmospherichumidity. In addition, the corrosive effect on metals (for example incircuit boards) which is frequently present in the case of chemicalantistatic agents is not observed in the case of the layers according tothe invention.

The working examples below are intended to illustrate the invention ingreater detail without representing a limitation to the embodimentsrepresented explicitly.

EXAMPLE 1

18 g of electroconductive polyethoxythiophene (described in Example 1 ofEP-A-0 257 573, specific electrical conductivity: 0.85 S/cm) and 102 gof a water-dispersible copolyester containing about 90 mol % ofisophthalic acid and 10 mol % of the sodium salt of 5-sulfoisophthalicacid as the acid component and 100 mol % of ethylene glycol as theglycol component (described in EP-A-0 144 878) were dissolved in 376 gof N-methylpyrrolidone (NMP) at 60° C. The solution was then addeddropwise with continuous stirring to 1488 g of water (80° to 90° C.)together with 10 g of ®Surfynol 104 (wetting agent from Air Products).The resultant dispersion has a total solids content of 7% by weight; therelative mixing ratio between the conductive polyethoxythiophene and thebinder used is 15:85. The mixing ratio between water and NMP is 80:20.

This dispersion was used to continuously coat a polyester film(Hostaphan RN 36, width 340 nm) by means of a smooth roller(counter-rotating with an air brush) and dried at 125° C. in acirculating-air tunnel).

The coated film has a surface resistance of 5×10⁷ ohms (DIN 53482) bymeans of a spring-tongue electrode), a transparency of 79% (ASTM1003-61) and a haze of 14.5% (ASTM 1003-61). The scratch resistance ofthe layer corresponds to pencil hardness 1H (ASTM 3363). Scanningelectron photomicrographs show a fineness of the dispersion particles inthe dried layer of on average less than 100 nm.

EXAMPLE 2

0.53 g of electroconductive polyethoxythiophene from Example 1 isdissolved at room temperature in 12.1 g of NMP, and 0.18 q of ®Surfynol104 is added. 20 g of ®Zinpol 1519 (an acrylic resin dispersed in waterto give a solids content of 24%, obtainable from Worlee-Chemie) isdiluted with 28 g of water; 0.12 g of ®Triton X100 (flow-control agent,Rohm und Haas) is subsequently added. The NMP solution is thenincorporated dropwise into the aqueous phase. The finished dispersioncontains 80% of water and 20% of NMP, the solids content is about 8.5%by weight, and the mixing ratio between polyethoxythiophene and ®Zinpol1519 is 1:9.

This dispersion was then applied to a corona-pretreated polyester film(Hostaphan RN 25) by means of a wire doctor and dried for 2 minutes at120° C. in a circulating-air drying cabinet. A surface resistance of3×10⁷ ohms and a transparency of 81% were achieved. The adhesion andwipe resistance were good, and the scratch resistance was 1H.

EXAMPLE 3

A 200×300 mm sample of a polyester film antistatically coated on oneside as in Example 1 (thickness 36 μm, transparency 79%, surfaceresistance of the coated side 5×10⁷ ohms) was adhesive-laminated with asingle-layer film of low density polyethylene (LDPE) having a thicknessof 50 μm. To this end, the uncoated side of the polyester film wascoated with a two-component polyurethane-based adhesive by means of a 12μm hand coater, placed together with the LDPE film and dried for oneminute at 40° C. in a circulating-air oven. The transparency of theresultant film laminate was 77%, and the bond strength, determined bymeans of a T-peel test, was 4 N/15 mm of strip width. Heat-sealed bagswere produced from the resultant film laminate, with the antistaticcoating on the polyester film being arranged on the outside of the bag.The polyethylene sides of the laminate were heat-sealed for 0.5 s at160° C. and 0.15 bar in a low-pressure heat-sealing unit. The sealstrength, determined by the T-peel method, was 25 N/15 mm of stripwidth.

We claim:
 1. An aqueous dispersion comprising electroconductive,polyalkoxythiophene which is soluble in aprotic organic solvents, saiddispersion having a residual organic solvent content of at least about5% by weight and less than or equal to 25% by weight, based on the totalweight of the dispersion.
 2. A dispersion according to claim 1, whereinsaid electroconductive polyalkoxythiophene comprises structural units ofthe formula ##STR2## in which R₁ is a C₁ -C₁₂ -alkoxy group, and themonomer units are bonded to one another in the 2-and/or 5-position.
 3. Adispersion according to claim 1, additionally containing at least onenonconductive binder which is dispersible or soluble in water.
 4. Adispersion according to claim 2, wherein said R₁ is a C₁ -C₄ -alkoxygroup.
 5. A dispersion according to claim 1, wherein said residualorganic solvent content is 10 to 20% by weight.
 6. A dispersionaccording to claim 1, wherein said residual organic solvent isN-methylpyrrolidone.
 7. A dispersion according to claim 1, wherein thewater content is 95 to 75% by weight.
 8. A dispersion according to claim7, wherein said water content is 80 to 90% by weight.
 9. A process forthe preparation of a dispersion according to claim 1, comprising thesteps of:(a) dissolving the electroconductive polyalkoxythiophene in atleast one dipolar, aprotic solvent; and (b) dispersing the solution inwater.
 10. A process for the preparation of a dispersion according toclaim 3 comprising the steps of:(a) dissolving the electroconductivepolyalkoxythiophene in at least one dipolar aprotic solvent; and (b)dispersing or dissolving said binder in said solvent; (c) dispersing thesolution in water.
 11. An aqueous dispersion comprisingelectroconductive, polyalkoxythiophene which is soluble in aproticorganic solvents, said dispersion having a residual organic solventcontent of at least about 5% by weight and less than or equal to 25% byweight, a water content of 95 to 75% by weight, and a solids content of2 to 15% by weight, based on the total weight of the dispersion.
 12. Adispersion according to claim 11, wherein said residual organic solventcontent is 10 to 20% by weight.
 13. A dispersion according to claim 11,wherein said solids content is 4 to 8% by weight.
 14. A dispersionaccording to claim 11, wherein said water content is 80 to 90% byweight.
 15. A dispersion according to claim 11, wherein said residualorganic solvent is N-methylpyrrolidone.
 16. A dispersion according toclaim 11, additionally containing at least one nonconductive binderwhich is dispersible or soluble in water.